Interferometer spectrometer with reduced alignment sensitivity

ABSTRACT

An interferometer spectrometer that has reduced alignment sensitivity is described herein. Parallelism of an output ray pair formed by a single input ray is not affected by variations in relative alignment of the components. In comparison to other compensated interferometer designs, lateral separation errors in the output ray pair due to optical component misalignment are reduced. The reduced alignment sensitivity may be accomplished by utilizing simple planar components that are common to both light paths. The reduced alignment sensitivity and simplicity in design provides a more compact and more robust interferometer, with reduced manufacturing costs associated therewith. An elliptical field of view light source that utilizes an array of collimator lenses is also described. The light source provides a more compact design than a single circular collimator lens of the same area, and is suitable for single channel or multi-channel use.

FIELD OF THE INVENTION

The present invention is generally related to interferometers.Specifically, the present invention is related to interferometers foruse as spectrometers, such as Fourier transform spectrometers.

BACKGROUND OF THE INVENTION

Interferometers have historically enjoyed a wide variety of applicationsfor analyzing material properties. For example, as incorporated in aFourier transform spectrometer, an interferometer may be used in themedical field to detect and measure various constituents of body tissuesand fluids. Interferometer spectrometers are particularly useful in themedical field because they allow for relatively non-invasive measurementtechniques, as compared to prior art techniques which require tissueand/or fluid sampling by physically removing the sample from thepatient.

The ability to perform relatively non-invasive procedures in themeasurement of body tissue and/or fluid characteristics provides atremendous advantage over the relatively invasive procedures of theprior art. For example, U.S. Pat. No. 5,830,132 to Robinson describes arobust and accurate non-invasive analyte monitor utilizing a lightdispersion device such as an interferometer spectrometer for themeasurement of blood constituents including glucose, alcohol, BUN (bloodurea nitrogen), bilirubin, hemoglobin, creatin, cholesterol, andelectrolytes. Another example of a non-invasive analyte monitor isdisclosed in U.S. Pat. No. 5,655,530 to Messerschmidt. The system andmethod of Messerschmidt '530 utilizes spectrographic techniques inconjunction with an improved optical interface. As applied to themeasurement of blood glucose levels, the analyte monitors disclosed inMesserschmidt '530 and Robinson '132 provide a diabetic patient with theopportunity to greatly improve control of the disease by more frequentor even continuous glucose monitoring, which translates into a reductionin diabetic related complications, an increase in patient comfort, anincrease in life expectancy, and an overall improvement in daily lifecoping with the disease.

Continuous or at least more frequent glucose monitoring is achieved byeliminating the necessity to obtain a blood or other fluid sample.Practically speaking, a blood sample may not be obtained on a continuousbasis nor at a sufficient frequency due to obvious reasons associatedwith risk of infection, patient discomfort, and patient lifestyle. Theanalyte monitors disclosed in Messerschmidt '530 and Robinson '132overcome these obstacles by providing a non-invasive and painless meansto measure blood glucose levels, thereby eliminating risk of infectionand patient discomfort.

From the foregoing, it is apparent that interferometer spectrometers mayhave a significant impact on continuing efforts to improve the health ofchronically ill patients, such as diabetics, by providing a significantimprovement over prior art systems and methods of analyzing bodilytissues and/or fluids. However, this and many other applicationsrequire, for practical purposes, a relatively compact and robustinterferometer. Specifically, a practical application of aninterferometer spectrometer requires that the interferometer be compactsuch that it is portable and robust such that it is able to withstandthe abuse of everyday use. Unfortunately, the prior art interferometersare, relatively speaking, neither sufficiently compact nor sufficientlyrobust to provide a practical interferometer spectrometer for portableuse.

Probably the most famous interferometer design is the Michelsoninterferometer, which is commonly used for Fourier transformspectroscopy. A form of Michelson interferometer commonly used forFourier transform spectroscopy includes six (6) basic components,namely, a collimated light source, a beam splitter, a compensator, afixed flat end mirror, a movable flat end mirror, and a light detector.The movable end mirror may be translated along an axis perpendicular toits surface to generate a series of optical path length differences(OPD) used to measure the spectral properties of the light.

In use, light emitted from the light source strikes the beam splitter,which partially reflects and partially transmits the light therethrough.The reflected beam travels to the movable mirror and is reflected backthrough the beam splitter toward the detector. The transmitted beamtravels through the compensator plate (same thickness and material asthe beam splitter plate) to the fixed end mirror and is reflected backthrough the compensator plate, reflected off of the beam splitter andtoward the light detector.

As mentioned previously, the movable mirror may be translated back andforth with a finely calibrated screw adjustment, or the like, togenerate an optical path length difference (OPD) or cause retardationsuch that the recombined beam forms an interference pattern, commonlyreferred to as an interferogram. Retardation is the OPD between a pairof output rays originating from a single input ray. By observing theinterference pattern, and measuring the distance the movable mirror istranslated, the wavelength of the light provided by the light source maybe determined. Further, changes in wavelength may be measured todetermine the index of refraction of test samples which may then be usedto identify the material and characteristics of the test sample. Furtheryet, by observing the interference pattern at various wavelengths, theamount of light absorbed by test sample may be measured, which isindicative of the material and properties of the test sample.

Although the Michelson interferometer is extremely useful, it tends tobe relatively sensitive to alignment of its various components. Inparticular, a tilt error is created by a change in the angle of the beamsplitter, the fixed-end mirror, or the movable-end mirror relative tothe other components. Tilt error may be defined as a deviation fromstrict parallelism of a pair of output rays originating from a singleinput ray. The effect of a tilt error is to reduce the modulationefficiency of the interferometer, in a wavelength dependent manner,causing a spectral calibration error. For example, a change in angle ofan end mirror, corresponding to an edge displacement (relative movementof opposite edges of the end mirror) by less than five percent (5%) ofthe wavelength of the light, causes an unacceptable change incalibration of the interferometer. This type of alignment sensitivity isparticularly difficult to eliminate with regard to the movable endmirror.

Attempts have been made, with limited success, to eliminate the tilterror of the Michelson interferometer by replacing the flat end mirrorwith retroreflectors as described by W. H. Steel, “Interferometers forFourier Spectroscopy,” Aspen International Conference on FourierSpectroscopy, pp. 43-53 (1970). Although replacing the flat end mirrorswith retroreflectors, such as cube-corner type or “cat's-eye” typeretroreflectors, eliminate tilt error, a shear error may be caused bythe lateral displacement of either retroreflector or a tilt of the beamsplitter. Shear error is the lateral displacement of one light pathrelative to the other light path which causes a wavelength dependentreduction in the modulation efficiency of the interferometer. Shearerror may be defined as a lateral separation of a pair of paralleloutput rays originating from a single input ray when the optical pathdifference (OPD) between the two rays is zero. Even a relatively smallshear error on the order of a few wavelengths of light may bedetrimental to the calibration of the interferometer.

Other attempts have been made to improve on the Michelson interferometerdesign in an effort to reduce alignment sensitivity of the components.For example, the Folded Jamin design provides a relatively stable designutilizing a relatively thick beam splitter plate and a rocking mirror asdescribed by L. Mertz, “Transformations in Optics,” page 50 (1965).Although the Folded Jamin design reduces component alignmentsensitivity, an exact ray trace analysis of the design demonstrates thatthe allowable field of view (FOV) is relatively small, particularly ascompared to the FOV of the Michelson interferometer. A relatively smallFOV renders the Jamin interferometer unsuitable for Fourier transformspectroscopy, particularly when the signal-to-noise ratio must beoptimized through the use of a light source of a large angular subtense.

Further attempts have been made to reduce the alignment sensitivity ofthe Michelson interferometer by rotating the interferometer componentsas a group to generate the OPD. For example, U.S. Pat. No. 4,684,255 toFord and the article by R. S. Sternberg and J. F. James “A New Type OfMichelson Interference Spectrometer,” J. Sci. Instru., Vol. 41 (1964)pp. 225-226, describe interferometers wherein the OPD is generated byrotating four components as a group. Another example is disclosed inU.S. Pat. No. 5,537,208 to Bertram et al. which describes aninterferometer wherein the OPD is generated by rotating two mirrors inparallel. Although tilt error and shear error are eliminated by thesedesigns to the extent that the components are rotated as a group with norelative movement therebetween, tilt and shear error may be caused by anincorrectly positioned component as constructed. As such, these designsinherently rely on the precise positioning and mounting of thecomponents, as constructed and maintained thereafter, to eliminate tiltand shear error. For example, European Patent Application 0681166 A1proposes the use of optically flat and parallel spacers to establishoptical contact between the critical components and thereby maintain theprecise position of the components. However, such component mountingtechniques are relatively costly to implement.

In sum, many of the interferometer spectrometers proposed in the priorart are sensitive to relative alignment between the critical components,and thus are susceptible to tilt error and/or shear error. Attempts toreduce the alignment sensitivity of the various components have been metwith limited success. Specifically, interferometer spectrometers of theprior art that reduce tilt and/or shear error have done so bycompromising other performance aspects of the design and by increasingmanufacturing costs.

SUMMARY OF THE INVENTION

The interferometer spectrometer of the present invention reducesalignment sensitivity of the critical components without compromisingperformance or increasing manufacturing costs. Specifically, as comparedto the Michelson interferometer, the interferometer of the presentinvention does not produce tilt error due to relative tilting of thecomponents. As compared to the modified Michelson interferometerutilizing retroreflectors, the interferometer spectrometer of thepresent invention greatly reduces shear error due to tilting or lateralmovement of any of the components. In addition, as compared to the Jamininterferometer, the interferometer spectrometer of the present inventionprovides a much larger FOV. Further, as compared to the component grouprotation interferometers, the interferometer spectrometer of the presentinvention eliminates tilt and shear sensitivity of the individualcomponents, as opposed to groups of components, thereby providing a morestable design with less complex and lower-cost component mountingtechniques. Further yet, as compared to prior art interferometerspectrometers that are field-widened, the interferometer spectrometer ofthe present invention is field-widened without introducing thepossibility of tilt and/or shear error.

The present invention overcomes the disadvantages of the prior art byproviding an interferometer spectrometer that has reduced alignmentsensitivity. In particular, variations in relative alignment (angular ortranslational displacement) do not adversely affect the parallelism(i.e., tilt error) of the recombined output ray pair, and thus do notresult in calibration error. In addition, translational variations inrelative alignment do not change the separation (i.e., shear error) ofthe output ray pair, and thus do not result in calibration error.Furthermore, rotational variations in relative alignment produce verylittle separation (i.e., shear error) of the output ray pair, and thusreduce sensitivity to mounting alignment and stability tolerances ascompared to a Michelson interferometer with cube-comer or “cat's-eye”retroreflectors. The reduced alignment sensitivity may be accomplishedby utilizing simple planar components that are common to both lightpaths. The reduced alignment sensitivity and simplicity in designprovides a more compact and more robust interferometer, with reducedmanufacturing costs associated therewith.

In an exemplary embodiment of the present invention, the interferometerspectrometer includes a beam splitter, a means for redirecting the splitback toward the beam splitter, and a means for generating a path lengthdifference (OPD) between the split rays. Both of the split raysoptically interact with each of the beam splitter, the redirectingmeans, and the means for generating a path length difference, therebyreducing alignment sensitivity. The split rays are recombined by thebeam splitter to form an output ray pair, wherein the rays forming theoutput ray pair are parallel. The interferometer may include acompensator, and the path length difference generating means maycomprise rotation of the beam splitter, the redirecting means, or thecompensator.

With this arrangement, translational and rotational changes in relativeposition between the beam splitter, the redirecting means, and the meansfor generating a path length difference do not result in a lack ofparallelism between the rays forming the output ray pair. Further,translational changes in relative position between the beam splitter,the redirecting means, and the means for generating a path lengthdifference do not result in a lateral separation of the rays forming theoutput ray pair. Further yet, there is no lateral separation of the raysforming the output ray pair when the first and second rays strike theend mirror at normal incidence.

In another exemplary embodiment of the present invention, theinterferometer spectrometer includes a beam splitter, an end mirror anda means for generating an optical path length difference (OPD). Theinterferometer may also include a compensator and a scanner platedisposed between the beam splitter and the end mirror. The beam splittercauses an input ray to be split into a first ray and a second ray havinga first path and a second path, respectively. The end mirror terminatesthe first and second paths to define a first path length and a secondpath length, respectively. The end mirror also reflects the first andsecond rays back to the beam splitter to combine the rays into an outputray pair. The OPD generating means causes a difference between the firstand second path lengths to create varying amounts of constructive ordestructive interference between the two output rays. The OPD may begenerated by rotating the beam splitter, the compensator, or the scannerplate. The output ray pair has a substantial degree of parallelism,which is independent of variations in the relative translational orangular position of the components and a separation which is independentof variations in the relative translational position of the components.Preferably, both the first and second rays are common to the beamsplitter and the compensator, and both rays reflect off one end mirror.The beam splitter and the compensator each preferably have a simpleplanar geometry such that the first and second rays are parallel to eachother after passing therethrough.

In yet another exemplary embodiment of the present invention, the lightsource for an interferometer spectrometer produces an elliptical angularsubtense. The elliptical angular subtense light source of the presentinvention provides an interferometer spectrometer having an increasedthroughput relative to an interferometer utilizing a light source ofcircular angular subtense. The light source may include a singlecollimator lens or an array of collimators lenses each having an arrayof transmitting fibers disposed adjacent an array of receiving fibers.The array of collimator lenses provides a more compact design than asingle circular collimator lens of the same area, and is suitable forsingle channel or multi-channel use.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram illustrating a first embodiment of aninterferometer in accordance with the present invention;

FIG. 2 is a schematic diagram illustrating a second embodiment of aninterferometer in accordance with the present invention;

FIG. 3 is a schematic diagram illustrating a third embodiment of aninterferometer in accordance with the present invention;

FIG. 4 is a schematic diagram illustrating an example of an opticalsystem incorporating the interferometer of the third embodiment inaccordance with the present invention;

FIG. 5A is a front view of an array of collimator lenses in accordancewith the present invention;

FIG. 5B is a side view of the array of collimator lenses illustrated inFIG. 5A; and

FIG. 5C is a rear view of an individual collimator lens in the array ofcollimator lenses illustrated in FIG. 5A.

DETAILED DESCRIPTION OF THE INVENTION

The following detailed description should be read with reference to thedrawings in which similar elements in different drawings are numberedthe same. The drawings, which are not necessarily to scale, depictillustrative embodiments and are not intended to limit the scope of theinvention.

Refer now to FIG. 1, which illustrates a schematic diagram of aninterferometer 10 for use as an interferometer spectrometer,particularly a Fourier transform spectrometer. The interferometer 10provides a compact and stable design that is relatively insensitive tosmall alignment errors of the various components. Such a compact andstable design has been shown to be particularly useful when incorporatedinto a spectrometer used for near infrared non-invasive measurement ofblood glucose and other blood analytes in humans, as described inMesserschmidt '530 and Robinson '132, the entire disclosures of whichare hereby incorporated by reference. Although described with referenceto an interferometer spectrometer used for non-invasive blood glucosemeasurement, the interferometers described herein are equally suitablefor other applications requiring a spectrometer having a spectralcalibration function insensitive to component alignment error.

Interferometer 10 includes three basic optical components, namely beamsplitter 12, compensator plate 14, and end mirror 16. Those skilled inthe art will recognize that through proper selection of materials,dimensions, and tolerances of the various components, the interferometer10 may be utilized for all wavelengths, from ultraviolet to longwaveinfrared. Generally, as used herein, an optical component may comprise ahomogenous piece of optical material which may be uncoated or coated onvarious parts of its surfaces with an anti-reflective coating, areflective or mirror coating, or a beam splitter coating. Also as usedherein, a single optical group may be used to describe an assembly ofoptical components. Those skilled in the art will recognize that some ofthe optical components discussed herein have known functionalsubstitutes that are not mentioned for purposes of brevity only.

A light source (not shown) provides an input ray at point A, and a lightdetector (not shown) collects an output ray at point N. Although it ispreferred to have a light source provide an input ray at point A andhave a light detector collect the output ray at point N, the lightsource and the light detector may be switched. In particular, a lightsource may provide an input ray at point N, and a light detector maycollect the output ray at point A. Furthermore, those skilled in the artwill recognize that light may be introduced and collected at the samepoint, either A or N, by utilizing a means to prevent interferencebetween the introduced light and the collected light. For example, thelight source and the light detector may be moved off axis in equal andopposite directions so that the input ray is separated from the outputray by a small angle.

As stated previously, the interferometer 10 includes a beam splitter 12and a compensator 14. The beam splitter 12 is an optical component whichhas as one of its functions the splitting of an input ray into twodifferent paths. The beam splitter 12 comprises a plate having a leadingedge or surface 12 a and a trailing edge or surface 12 b. The leadingsurface 12 a is parallel to the trailing surface 12 b. The beam splitter12 comprises a homogenous refractive material which is transparent inthe wavelength region of interest. Beam splitter 12 includes a partiallyreflective coating 12 c deposited on a portion of the trailing surface12 b and a completely reflective coating 12 d deposited on a portion ofthe leading surface 12 a. The partially reflective coating 12 c splitsthe input ray into two separate rays of approximately equal intensity,wherein one ray is transmitted and the other ray is reflected. Thecompletely reflective coating 12 d reflects all light striking it fromwithin the plate 12.

The compensator 14 is an optical component which has as one of itsfunctions the equalization of optical path length through the variousrefractive materials disposed in two optical paths. The compensatorplate 14, similar to beam splitter 12, comprises a plate having aleading edge or surface 14 a and a trailing edge or surface 14 b. Theleading surface 14 a is parallel to the trailing surface 14 b. Thecompensator plate 14 preferably comprises the same homogenous refractivematerial of preferably the same thickness as used in beam splitter plate12, such that both beams are equally influenced by refraction. Acompletely reflective coating 14 d is deposited on a portion of leadingsurface 14 a and a completely reflective coating 14 c is deposited on aportion of trailing surface 14 b. Completely reflective coatings 14 cand 14 d reflect all light striking the coatings from inside the plate14.

If the compensator plate 14 has a different thickness than beam splitter12, the light rays will travel through different amounts of air anddifferent amounts of refractive material. This difference in materialresults in phase error which manifests itself as an asymmetry in theinterference pattern. To correct moderate amounts of phase error, whichis wavelength dependent because the refractive index of the materialvaries as a function of wavelength, mathematical post processingtechniques may be utilized. Such mathematical post processing techniquesare known in the art, and thus the plate thickness of the compensator 14relative to the beam splitter 12 need not be precisely equal.

End mirror 16 is a simple flat mirror arranged orthogonally with theinput ray from the light source (not shown) such that the split rays arereversed in direction and travel back to the beam splitter to combinethe split rays into an output ray pair. Preferably, the end mirror 16 isfixed and is common to both of the split rays. Utilizing one end mirror16, as opposed to two end mirrors for each of the individual rays,simplifies the construction of the interferometer 10 and reduces thepotential for alignment variations between the components. Those skilledin the art will recognize that a wide variety of means for redirectingthe first and second rays back toward the beam splitter 12 may be usedin place of end mirror 16. For example, the end mirror 16 could bereplaced by a Porro prism, a V-mirror or a retroreflector (e.g., cat'seye or cube comer) to redirect the first and second rays back toward thebeam splitter 12.

A Porro prism and a V-mirror are optical components including twoperpendicular mirror surfaces. If a Porro prism or V-mirror ispositioned such that the vertex (i.e., the line of intersection of thetwo mirror surfaces) is parallel to the line intersecting the first andsecond rays, then the system will function as with the flat end mirror16, albeit with reduced FOV and a potential for shear error.

If a retroreflector is used as a replacement for the flat end mirror 16,preferably a single retroreflector is employed because the use of tworetroreflectors may introduce the potential for shear error. A singleretroreflector intercepting both the first and second rays may providean interferometer with no tilt or shear error. For example, if theretroreflector is positioned well above or below the centerline of theaxial rays such that all outgoing rays from the retroreflector arephysically separated from the incoming rays, and the output rays passthrough a complimentary (i.e., all components are flipped over to looklike the mirror image of the components on the incoming side) system ofa compensator, an OPD scanner and a beam splitter, then theinterferometer may have no tilt or shear error.

For purposes of illustration only, a single ray trace is illustratedthrough the interferometer 10. Those skilled in the art will recognizethat the interferometer 10 is suitable for both single channel andmultichannel use by stacking the channels (i.e., rays) in a directionperpendicular to the plane of the illustration in FIG. 1. For ease ofidentification, the individual ray segments have been labeled withcapital letters, starting with the input ray AB. Input ray AB enters thebeam splitter plate 12 at leading surface 12 a resulting in refractedray BC. Refracted ray BC is then split into a first ray path initiallydefined by ray CD and a second ray path initially defined by ray CI.

Beginning with the first path, ray CD is reflected off reflectivecoating 12 d on surface 12 a to form ray DE. Ray DE is refracted atpoint E and enters the compensator plate 14 at point F on leadingsurface 14 a. Ray EF is refracted at point F to form ray FG, which inturn is refracted at point G to form ray GH. Ray GH is reflected offmirror 16 and returns along the same path. Thus, the first path isdefined by ABCDEFGH.

The second path, as initially defined by ray CI, enters compensatorplate 14 through leading surface 14 a at point I and is refracted toform ray IJ. Ray IJ is reflected off reflective coating 14 c to form rayJK. Ray JK, in turn, is reflected off reflective coating 14 d to formray KL. Ray KL is refracted upon passing through trailing surface 14 bat point L to form ray LM. Ray LM is reflected off of the end mirror 16and returns is along the same path. Thus, the second path is defined byABCIJKLM.

Utilizing the ray trace analysis provided above, a number ofobservations may be made about the interferometer 10. For example, eachof the first and second rays optically interact with each of the opticalcomponents. As used herein, optically interact may be used to describeany condition where an optical ray interacts with an optical componentto undergo reflection or refraction. By optically interacting with allof the components, the interferometer 10 has reduced alignmentsensitivity for reasons set forth in more detail hereinafter.

Several other observations may be made by virtue of the ray traceanalysis provided above. Assuming the leading surface 12 a of the beamsplitter 12 is parallel to the trailing surface 12 b, rays EF and CIwill be parallel, regardless of variations in angle between the beamsplitter 12 and the input ray AB. Similarly, assuming the leadingsurface 14 a of the compensator plate 14 is parallel to the trailingsurface 14 b, ray CI will be parallel to ray LM and ray EF will beparallel to ray GH. Further, ray GH will be parallel to ray LMindependent of relative position between the beam splitter 12, thecompensator plate 14 and the end mirror 16.

The first cumulative ray corresponding to the first path and the secondcumulative ray corresponding to the second path are recombined at pointC on the trailing surface 12 b of the beam splitter 12 and emerge as anoutput ray pair comprising two rays in parallel. An output ray pairemerges as both ray BA and CN. The parallelism of the recombined rays isthus dependent only on the flatness and parallelism of the surfaces ofthe individual components of the interferometer 10. Specifically, theparallelism of the recombined rays is not dependent on variations inrelative placement of one component relative to the other components ofthe interferometer 10. An error in parallelism between the tworecombined rays would otherwise produce a wavelength dependent reductionin modulation efficiency of the interferometer 10, thus affectinginstrument calibration. An error in parallelism between the individualrays forming the recombined ray pair is commonly referred to as tilterror.

Specifically, tilt error may be defined as the lack of parallelismbetween a pair of output rays (such as ray pair BA or ray pair CN)generated by one input ray (such as ray AB). The tilt error or anglebetween the rays forming the output ray pair, in the present invention,depends only on the angle between the two surfaces 12 a and 12 b of thebeam splitter 12, the angle between the two surfaces 14 a and 14 b ofthe compensator plate 14 (if a compensator is utilized), and theflatness of all optical surfaces of the components comprising theinterferometer 10. The two rays forming the output ray pair will beparallel if the two beam splitter 12 surfaces 12 a and 12 b areparallel, the two compensator plate 14 surfaces 14 a and 14 b areparallel, and all optical surfaces are flat. If these conditions aresatisfied during the manufacture, mounting and assembly of thecomponents, the result is zero tilt error, assuming the rays strike theintended surfaces of the components in the intended sequence, asillustrated. Because the parallelism of the recombined rays isindependent of changes in placement of the components relative to eachother, the interferometer 10 is fully tilt compensated.

The interferometer 10 of the present invention is also partially shearcompensated. Shear error may be defined as the lateral separation of twoparallel rays forming the output ray pair when the optical path lengthdifference is zero. Lateral separation refers to separation in adirection perpendicular to the path of propagation of the output raypair. Thus, when the tilt error is zero, shear error is the lateralseparation between the individual rays forming the output ray pair. Zeroshear error is accomplished when there is no lateral separation betweenthe rays forming the output ray pair.

In the present invention, relative translational changes in position ofthe components comprising the interferometer 10 do not cause shear oraffect shear error. Relative angular changes in position of thecomponents of the interferometer 10 do affect shear error. However,relative to a Michelson interferometer with retroreflectors, relativelylittle shear error is introduced by angular changes in position of thecomponents.

A properly adjusted Michelson interferometer with flat end mirrors willhave zero shear error. However, as mentioned previously, such aMichelson interferometer is susceptible to tilt error absent the use ofretroreflectors. On the other hand, a Michelson interferometer withcube-comer or “cat's-eye” retroreflectors to compensate for tilt errorwill have a large shear error equal to twice the lateral displacement ofone of the retroreflectors from an optical center line established bythe other retroreflector. By contrast, the interferometer 10 of thepresent invention, which is fully tilt compensated, has very littleshear error.

Thus, the interferometer 10 is partially shear compensated relative to aMichelson interferometer with retroreflectors, thereby relaxing thetranslational and angular positional tolerance requirements of thecomponents. Further, the interferometer 10 may be fully shearcompensated by adjusting the end mirror 16 such that the output ray pairis parallel to the input ray, provided the two individual rays formingthe output ray pair are parallel (i.e., zero tilt error). In thisregard, the interferometer 10 of the present invention has exceptionalinterferometric optical tolerance. Interferometric optical tolerance maybe defined as any rotational or transitional movement or surfaceirregularity that results in displacement of any part of the activeoptical area such that interferometer performance is affected. Areasonably good standard optical tolerance for interferometers is thedistance equal to one-tenth ({fraction (1/10)}) the wavelength of theshortest wavelength to be effectively measured by the interferometer.

Interferometer 10 may be used as a spectrometer by generating aninterference pattern between the split rays. An interference pattern,i.e., an interferogram, is created by generating an optical path lengthdifference (OPD) between the two paths, namely the first path ABCDEFGHand the second path ABCIJKLM. Since ray AB and ray BC are common to bothpaths, the OPD originates on the trailing surface 12 b of the beamsplitter 12 at point C.

Assuming the light source is provided at point A and the input ray ABforms an angle of incidence θ₁ with surface 12 a, and that thecompensator plate 14 is positioned relative to the beam splitter 12 suchthat ray CI forms an incident angle θ₂ equal to θ₁ , and furtherassuming that the end mirror 16 is positioned such that ray LM and rayGH strike the end mirror 16 at normal incidence, the optical path lengthCDEFGH is equal to the optical path length CIJKLM. Thus, the OPD underthese conditions is zero at all wavelengths.

An OPD (other than zero) may be generated by rotating any one of thethree primary components 12, 14, 16 of the interferometer 10 about theaxis perpendicular to the plane of the drawing shown in FIG. 1.Preferably, the phase compensator plate 14 is rotated as indicated byarrow 18 because it is the only component for which both rays BA and CN(i.e., the recombined rays) do not change angle as the compensator plate14 is rotated. It is desirable to avoid changing the angle of therecombined rays BA and CN such that the light source and light detectormay remain stationary.

When the phase compensator plate 14 is rotated as indicated by arrow 18,a slight change in length of the optical path CDEFGH is encountered anda relatively large change in length of the optical path CIJKLM isencountered. The OPD with respect to path CDEFGH and path CIJKLM, whichis proportional to the angle of rotation as indicated by arrow 18, iscaused, in part, by changes in the angle of refraction and reflectionthrough the compensator plate 14. Specifically, optical path CIJKLMencounters two reflections at points J and K resulting in a longer paththrough the compensator plate 14, and thus encounters a larger change inpath length when the angle of the compensator plate 14 is changed.Accordingly, by subtracting path length CDEFGH from path length CIJKLM,a net path length difference is obtained which is proportional to theangle of rotation of compensator 14 as indicated by arrow 18.

Although the net path length difference change is proportional to theangle of rotation of the compensator plate 14, the OPD versus rotationangle will vary slightly as a function of wavelength due to thedispersive nature of the material of the compensator 14. Specifically,as the compensator plate 14 is rotated from the position where OPDequals zero, the path length through the refractive material of thecompensator 14 increases in one direction of rotation and decreases inthe other direction of rotation. This variation manifests itself byproducing a slightly asymmetric interferogram, but is not considered aserious problem because it may be corrected by post processing of thedata by known mathematical processing techniques.

Mathematical post processing techniques may also be utilized tocompensate for differences in the amount of refractive materialencountered by both light paths resulting from misalignment of the endmirror 16. Specifically, if the end mirror 16 is tilted such that ray LMand ray GH do not strike the mirror 16 at normal incidence, the phasecompensator plate 14 must be rotated from the θ_(2=θ) ₁ position to findthe position where the OPD equals zero. However, by rotating thecompensator plate 14 to account for misalignment of the end mirror 16,an unbalance in the equality of refractive material in both light pathsis created. The unbalance of equality results in an optical path lengthdifference that may be accounted for by post data processing, usingknown mathematical processing techniques, without affecting thecalibration of the interferometer 10. Preferably, autocollimationtechniques, as known to those skilled in the art, may be utilized tominimize the misalignment of the end mirror 16.

In use, the interferometer 10 generates an interferogram which isrecorded by measuring the intensity of the light on the detector (notshown) at periodic intervals as the OPD is varied through a range ofvalues depending on the required measurement resolution. The actual OPDmust be known at each measurement position to within a small fraction ofthe wavelength of the light being measured. This is typicallyaccomplished by simultaneously recording the interferogram of anauxiliary reference beam from a monochromatic source of accurately knownwavelength, such as a HeNe laser. As applied to interferometer 10, sucha reference beam may be placed adjacent to the test beam. Alternatively,the reference beam may be introduced at point N, assuming the test beamis introduced at point A, or visa versa. Further details on the use ofthe interferometer 10 as applied to an interferometer spectrometer usedfor non-invasive measurement of analytes, may be found in Messerschmidt'530 and Robinson '132.

Refer now to FIG. 2, which illustrates a schematic diagram of analternative interferometer 20 in accordance with another aspect of thepresent invention. Except as described hereinafter, interferometer 20 isthe same in form and function as interferometer 10 described withreference to FIG. 1. With the rearrangement of the components ofinterferometer 20, the compensator plate 14 may have a smaller width forgiven collimated beam width as compared to interferometer 10.

In addition to the rearrangement of components, the interferometer 20utilizes a compensator 14 having a slightly different arrangement ofreflective coatings 14 c and 14 d. Interferometer 20 defines a firstlight path CDEFG and a second light path CIJKH. Ray CI is parallel toray EF if the leading surface 12 a of beam splitter 12 is parallel tothe trailing surface 12 b. Similarly, assuming that ray CI is parallelto ray EF, ray KH will be parallel to ray FG provided that leadingsurface 14 a of compensator plate 14 is parallel to trailing surface 14b. As with interferometer 10, the return rays of interferometer 20remain parallel independent of the position of the components relativeto each other.

Also similar to interferometer 10, the beam splitter 12 or thecompensator plate 14 of interferometer 20 may be rotated to generate theOPD. Alternatively, in order to avoid changing the angle of the outputray BA or CN, a scanner plate 22 (shown in phantom) may be provided. Thescanner plate 22 comprises a thin plate of refractive material and ispositioned approximately at the intersection of ray CI and ray FG.Rotation of the scanner plate 22 about an axis perpendicular to theplane of the drawing causes the amount of refractive material in onepath to increase while decreasing the amount of refractive material inthe other path. Thus, rotation of the scanner plate 22 causes an OPD.Under perfect alignment conditions, zero OPD is achieved when the angleof incidence of both light paths is equal. Variation in the incidenceangle caused by rotation of the scanner plate 22 causes a correspondingand proportional change in OPD.

Scanner plate 22 may be positioned at the intersection of rays CI and FGas illustrated. Positioning the scanner plate at the intersection is notabsolutely necessary, but allows the scanner 22 to be made very small.In addition, manufacturing defects tend to be canceled out if thescanner is common to both light paths, such as at the intersection ofrays CI and FG. Further, commonality to both light paths provides moreOPD change per change in angle of rotation of the scanner 22.

In addition to the advantage of maintaining a constant angle of theoutput ray BA or CN, utilizing the scanner plate 22 to generate the OPDprovides the advantage of a symmetrical interferogram and rapidscanning. A symmetrical interferogram is achieved by eliminatingdifferences in the amount of refractive material between the two lightpaths, other than as used to generate the OPD. The rapid scanning isachieved by virtue of the low mass of the scanner plate 22, which isrelatively small as compared to the beam splitter 12 and the compensatorplate 14. In addition, a very thin scanner plate 22 will produce arelatively small OPD change for a relatively large rotational anglechange of the plate 22. Thus, by utilizing a very thin scanner plate 22,the desired scanning may be achieved by continuously rotating the plate22. Furthermore, the relatively large angle change corresponding to therelatively small OPD change may eliminate the need for an OPD referencelaser beam, which may be replaced by a relatively simple shaft rotaryposition encoder.

Refer now to FIG. 3, which illustrates a schematic diagram of analternative interferometer 30 in accordance with yet another embodimentof the present invention. Except as described herein, interferometer 30is the same in form and function as interferometer 20 described withreference to FIG. 2. Interferometer 30 has a slightly differentarrangement of components to define a first light path CDEFGH and asecond light path CIJKLM. In addition to the rearrangement ofcomponents, interferometer 30 includes mirror 32 which creates anadditional intersection of the two light paths such that the scannerplate 22 may be repositioned. Thus, the space available to insert therefractive OPD scanner plate 22 is increased, without vignetting. Inpractice, this allows the beam splitter 12 and the phase compensatorplate 14 to be made with reduced thickness, thereby reducing the overallsize and cost of the interferometer 30, despite the use of an additionalcomponent, namely mirror 32.

In order to optimize the performance of the interferometer 30, a lightsource producing an elliptical angular subtense is preferred. A lightsource providing an elliptical angular subtense is described withreference to FIGS. 5A through 5C. This may be appreciated by performinga detailed ray trace analysis of the interferometer 30 whichdemonstrates that the allowable angular subtense, as compared to theMichelson interferometer, is much larger in a plane perpendicular to thedrawing and somewhat smaller in a plane parallel to the drawing. Theallowable angular subtense may be defined as the light source angularsubtense for which the interferogram produced by a ray originating atthe edge of the light source is 180 degrees out of phase with that of aray originating from the center axis of the light source. The allowableangular subtense is measured at a specific wavelength, generally theshortest wavelength of interest at the maximum retardation. Maximumretardation is set by the spectral resolution requirement of themeasurement.

The throughput of the interferometer 30 is proportional to the productof the two orthogonal angular subtense angles mentioned above.Throughput may be defined as the allowable solid angle of the lightsource, as viewed through the collimating lens, multiplied by thecollimated beam area at the exit pupil of the collimating lens. In otherwords, optical throughput is the product of the light source area andthe light source solid angle utilized by the interferometer. Theallowable solid angle is calculated from the allowable angular subtenseas defined above. Accordingly, the throughput of the interferometer 30is moderately larger than the throughput of the Michelsoninterferometer. By increasing the throughput of the interferometer 30,the interferometer becomes field widened. A field-widened interferometerallows the use of a smaller diameter collimator lens while maintainingthe required measurement signal-to-noise ratios. This is particularlyuseful when the interferometer 30 is adapted for multi-channel use.

For example, an analysis of a working model of interferometer 30 yieldedan allowable angular subtense of 0.62 times that of the Michelsoninterferometer in a plane parallel to the drawing and 2.8 times that ofthe Michelson interferometer in a plane orthogonal to the drawing,resulting in a total throughput improvement of approximately 1.8 timesthat of the Michelson interferometer. To take advantage of thisincreased throughput, a light source providing an elliptical angularsubtense is preferred. For example, in the working model ofinterferometer 30, elliptical angular subtense is preferred having aratio of approximately 4.6:1 for the major and minor axes. Those skilledin the art will recognize that the exact values of these parametersdepend on the construction parameters and the refractive index of thematerials used.

As with interferometers 10 and 20 described with reference to FIGS. 1and 2, respectively, interferometer 30 maintains parallelism of theoutput ray pair independent of the relative position of the components.Specifically, when ray AB is made parallel to ray BA by adjusting theend mirror 16, there is no sheer error, independent of the relativealignment of the other components. Although misalignment of the endmirror resulting in an angular difference between ray AB and ray BAintroduces some shear at zero OPD, the amount of sheer introduced isrelatively small as compared to a modified Michelson interferometerutilizing a cube-comer retro reflector. For example, an analysis of theworking model of interferometer 30 revealed that when ray AB and ray BAwere separated by approximately 3 degrees, the positional toleranceallowed for one of the phase compensator support points was about 30times greater than that allowed for lateral displacement of acube-corner or “cat's-eye” retroreflector. Accordingly, theinterferometer 30 is relatively sheer compensated.

Refer now to FIG. 4, which illustrates a schematic diagram of an opticalsystem 40 incorporating the interferometer 30 in accordance with thepresent invention. Optical system 40 incorporates a unit magnificationrelay design which is used to image the exit pupil of the collimatorlens 50 onto the surface of the end mirror 16. The unit magnificationrelay design minimizes the size of the critical interferometercomponents by minimizing the beam diameter throughout the system 40.Those skilled in the art will recognize that many unit magnificationrelay designs may be utilized, including both reflecting and refractingtype designs known in the art. For purposes of illustration only, therelay used in the optical system 40 includes two primary components,namely a concave mirror 44 and a convex mirror 46. This relayarrangement is described in detail by Offner in an article entitled “NewConcepts In Projection Mask Liners,” Opt. Eng., 14:131 (1975).

Optical system 40 further includes an OPD reference subsystem includinglaser 43, beam splitter 45, and detector 47. In use, the laser 43 andthe detector 47 are slightly offset from each other on opposite sides ofthe optical axis in order to prevent laser energy feedback after thelaser passes through the interferometer 30.

Optical system 40 further includes a light source 41 and a lightdetector 42. The light source 41 includes an incandescent lamp which isoptically coupled by fiber bundle 48 a to the sample 100 to be tested.Specific methods of optically coupling the sample 100 for purposes ofmeasuring blood constituents are described in U.S. Pat. No. 5,355,880and 5,830,132, the entire disclosures of which are hereby incorporatedby reference. Fiber bundle 48 b is positioned in the focal plane ofcollimator lens 50 to provide an input ray. Fiber bundle 48 c ispositioned in the focal plane of collimator lens 50 to collect an outputray from the interferometer 30 and transmit the output ray to thedetector 42. Thus, optical system 40 demonstrates how the interferometer30, in addition to interferometers 10 and 20, may be utilized asspectrometers.

Refer now to FIGS. 5A through 5C which illustrate various views of acollimator lens 50 and an array 56 of collimator lenses 50 in accordancewith the present invention. As mentioned previously, in order tooptimize the throughput of interferometer 30, it is preferable to use alight source that produces an elliptical FOV. Collimator lens 50 and/orarray 56 may be used to generate an elliptical FOV.

Allowable field-of-view (FOV) of the interferometer is the maximumangular subtense of the light source, viewed through the interferometer,for which all rays collected from the light source, having traveled oneof the two paths through the interferometer, have an OPD relative toeach other of less than or equal to one half the wavelength of the lightbeing measured. This definition applies at all retardation values up tothe retardation required to achieve the desired instrument spectralresolution. The allowable optical throughput is the optical throughpututilizing the allowable field-of-view as defined above. The termsmaximum angular subtense and allowable field-of-view areinterchangeable. The angular subtense of the light source is measured asthe angle the ray makes with an axis defined as the average direction ofpropagation of all input rays. For the designs of the current invention,it is found that the maximum angular subtense allowed by theinterferometer varies as a function of the azimuthal angle; i.e., theangle of the ray projected onto a plane perpendicular to the directionof propagation. The shape of this function is, in general, elliptical.

With specific reference to FIG. 5C which illustrates a rear view of anindividual collimator lens 50, a total of six optical fibers a, b, c, d,e, and f are positioned in the collimator focal plane. Fibers a, b, andc are transmit fibers 52, and fibers d, e, and f are receiver fibers 54.The fibers are positioned such that light emitted from fiber a iscollected by fiber f; light emitted from fiber b is collected by fibere; and light emitted from fiber c is collected by fiber d. Transmittingfiber array 52 corresponds to fiber bundle 48 b and receiving fiberarray 54 corresponds to fiber bundle 48 c, as illustrated in FIG. 4.

With this arrangement, the FOV in the vertical direction is three timesthe FOV in the horizontal position. Although a 3:1 ratio of fibers isillustrated, those skilled in the art will recognize that other ratiosmay be used as well. For example, it was determined that for the workingmodel of interferometer 30 as incorporated into optical system 40, theoptimum ratio was approximately 4.6:1. Generally, an array oftransmitting fibers 52 disposed adjacent an array of receiving fibers 54having a ratio greater than 1:1 will result in an elliptical FOV.

In order to provide a side-by-side arrangement of transmitting fibers 52adjacent receiving fibers 54 as illustrated in FIG. 5C, the transmittingfibers 52 must be separated from the receiving fibers 54 by a smallangle. For example, for the working model of interferometer 30 asincorporated into optical system 40, the transmitting fibers 52 wereseparated from the receiving fibers 54 by an angle of approximately 2.8degrees. This angular separation may be eliminated by using path AB forthe input and path CN for the output, or visa versa, as describedpreviously.

Refer now to FIGS. 5A and 5B which illustrate an array 56 of individualcollimator lenses 50 arranged in a parallel column. Each of thecollimator lenses 50 of the array 56 may be constructed as describedwith reference to FIG. 5C. Specifically, each collimator lens 50 in thearray 56 may include a plurality of transmitting fibers 52 disposedadjacent a plurality of receiving fibers 54. For multichannel operation,the transmitting fibers and the receiving fibers of each channel may goto separate light sources and separate light detectors. Alternatively,all of the transmitting fibers may be connected to a single light sourceand all of the receiving fibers may be connected to a single lightdetector for single channel operation. The arrangement of a plurality ofcollimator lenses 50 in an array 56 provides a more compact design thanwould be accomplished utilizing a single collimator lens of equivalentarea for single channel use.

The multi-channel use as permitted by array 56 is particularly usefulwhen multiple samples 100 are presented simultaneously for analysis. Forexample, process monitoring of a wide strip of material, where thelinear array 56 is set up perpendicular to the direction of the feed ofthe material, permits the interferometer to detect variations along thewidth of the material. In addition, simultaneous analysis of a number ofsamples with properties that change with time is made possible bymultichannel array 56. Further, multichannel array 56 provides increasedmeasurement throughput in situations here samples are available only fora specific period of time, such as on a conveyor which indexes samplesto a new position at specific time intervals. In this situation,throughput is increased by placing samples in parallel across theconveyor to line up with the individual lenses 50 of the array 56. Insum, the array 56 optimizes the shape and size of the collimated beamused in the interferometer 30, thereby allowing the interferometer 30component shape and size to be optimized for minimum production cost andmaximum size reduction.

From the foregoing, it should be apparent to the reader that the presentinvention provides a number of interferometer designs that provideinterferometer spectrometers with improved performance and ease ofmanufacture. The interferometers of the present invention have greatlyreduced sensitivity to alignment errors of the individual components ascompared to prior art interferometers. This allows calibration to bemaintained even under conditions in which small changes in angle andposition of the individual components occur. The practical advantages ofthis feature are that the component mounts may be simplified therebyreducing manufacturing costs and that the interferometer is more robustand compact, and thus more suitable for a portable application.

Those skilled in the art will recognize that the present invention maybe manifested in a variety of forms other than the specific embodimentsdescribed and contemplated herein. Accordingly, departures in form anddetail may be made without departing from the scope and spirit of thepresent invention as described in the appended claims.

What is claimed is:
 1. An interferometer spectrometer, comprising: abeam splitter for splitting an input ray into a first ray and a secondray having a first path and a second path, the first ray and the secondray exiting the beam splitter in parallel; a means for redirecting thefirst and second rays back toward the beam splitter to define a firstpath length and a second path length, respectively, the first ray andthe second ray striking the redirecting means in parallel; and a meansfor generating a difference between the first and second path lengths,wherein the first ray and the second ray optically interact with thebeam splitter, the redirecting means, and the means for generating apath length difference.
 2. An interferometer spectrometer as in claim 1,wherein the beam splitter combines the redirected first and second raysinto an output ray pair, and wherein the rays forming the output raypair are parallel.
 3. An interferometer spectrometer as in claim 2,wherein translational and rotational changes in relative positionbetween the beam splitter, the redirecting means, and the means forgenerating a path length difference do not result in a lack ofparallelism between the rays forming the output ray pair.
 4. Aninterferometer spectrometer as in claim 3, wherein translational changesin relative position between the beam splitter, the redirecting means,and the means for generating a path length difference do not result in alateral separation of the rays forming the output ray pair.
 5. Aninterferometer spectrometer as in claim 4, wherein there is no lateralseparation of the rays forming the output ray pair when the first andsecond rays strike the redirecting means at normal incidence.
 6. Aninterferometer spectrometer as in claim 1, wherein the beam splitter hasflat and parallel leading and trailing surfaces with homogenousrefractive material disposed therebetween.
 7. An interferometerspectrometer as in claim 6, further comprising a compensator disposedbetween the beam splitter and the redirecting means for equalizing theoptical path lengths of the first and second paths, the compensatorhaving flat and parallel leading and trailing surfaces with homogenousrefractive material disposed therebetween.
 8. An interferometerspectrometer, as in claim 6, wherein the beam splitter and thecompensator form an angle of intersection, and wherein the angle ofintersection is greater than zero degrees.
 9. An interferometerspectrometer as in claim 6, wherein the path length differencegenerating means comprises rotation of the beam splitter, theredirecting means, or the compensator.
 10. An interferometerspectrometer as in claim 9, wherein the path length differencegenerating means comprises rotation of the beam splitter.
 11. Aninterferometer spectrometer as in claim 9, wherein the path lengthdifference generating means comprises rotation of the redirecting means.12. An interferometer spectrometer as in claim 9, wherein the pathlength difference generating means comprises rotation of thecompensator.
 13. An interferometer spectrometer as in claim 1, furthercomprising a scanner plate of refractive material having parallel sides.14. An interferometer spectrometer as in claim 13, wherein the pathlength difference generating means comprises rotation of the scannerplate.
 15. An interferometer spectrometer, as in claim 1, wherein therays forming the output ray pair produce an elliptical angular subtensewithin the allowable field of view.
 16. An interferometer spectrometer,as in claim 1, wherein the rays forming the output ray pair have amaximum vertical angular subtense different from a maximum horizontalangular subtense within the allowable field of view.
 17. An opticalsystem, comprising: an interferometer spectrometer as in claim 1; alight source generating the input ray, the light source opticallyconnected the interferometer; a means for introducing one or more testsamples into the input ray, the introducing means disposed between thelight source and the interferometer; and a light collector for receivingthe output ray pair, the light collector optically connected theinterferometer.
 18. An interferometer spectrometer, comprising: a beamsplitter for splitting an input ray into a first ray and a second rayhaving a first path and a second path, the beam splitter having flat andparallel leading and trailing surfaces with homogenous refractivematerial disposed therebetween; a flat end mirror terminating the firstpath and the second path to define a first path length and a second pathlength, respectively, wherein the end mirror reflects the first andsecond rays back to the beam splitter to combine the rays into an outputray pair, wherein the rays forming the output ray pair are parallel; acompensator disposed between the beam splitter and the end mirror forequalizing the optical path lengths of the first and second paths, thecompensator having flat and parallel leading and trailing surfaces withhomogenous refractive material disposed therebetween; and a means forgenerating a difference between the first and second path lengths tocreate an interference pattern in the output ray pair, wherein the pathlength difference generating means is common to both the first andsecond paths.
 19. An interferometer spectrometer as in claim 18, whereintranslational and rotational changes in relative position between thebeam splitter, the compensator and the end mirror do not result in alack of parallelism between the rays forming the output ray pair.
 20. Aninterferometer spectrometer as in claim 18, wherein there is no lateralseparation of the rays forming the output ray pair when the first andsecond rays strike the end mirror at normal incidence.
 21. Aninterferometer spectrometer as in claim 18, wherein translationalchanges in relative position between the beam splitter, the compensatorand the end mirror do not result in a lateral separation of the raysforming the output ray pair.
 22. An interferometer spectrometer as inclaim 18, wherein the input ray is provided by a light source, andwherein the light source produces an elliptical angular subtense.
 23. Aninterferometer spectrometer as in claim 22, wherein the light sourceincludes a collimator lens having an array of transmitting opticalfibers disposed adjacent an array of receiving optical fibers.
 24. Aninterferometer spectrometer as in claim 23, wherein the lenses arearranged in parallel.
 25. An interferometer spectrometer as in claim 22,wherein a single channel is utilized.
 26. An interferometer spectrometeras in claim 22, wherein multiple channels are utilized.
 27. Aninterferometer spectrometer, comprising: a beam splitter for splittingan input ray into a first ray and a second ray having a first path and asecond path, the beam splitter comprising a plate having flat andparallel surfaces; a flat end mirror terminating the first path and thesecond path to define a first path length and a second path length,respectively, wherein the end mirror reflects the first and second raysback to the beam splitter to combine the rays into an output ray pair,wherein the rays forming the output ray pair are parallel; a compensatordisposed between the beam splitter and the end mirror for equalizing theoptical path lengths of the first and second paths, the compensatorcomprising a plate having flat and parallel surfaces; and a means forgenerating a difference between the first and second path lengths tocreate an interference pattern in the output ray pair, wherein the pathlength difference generating means is common to both the first andsecond paths; wherein translational changes in relative position betweenthe beam splitter, the compensator and the end mirror do not result in alack of parallelism or a lateral separation between the rays forming theoutput ray pair.
 28. An interferometer spectrometer, comprising: a beamsplitter for splitting an input ray into a first ray and a second rayhaving a first path and a second path, respectively; an end mirrorterminating the first path and the second path to define a first pathlength and a second path length, respectively, wherein the end mirrorreflects the first and second rays back to the beam splitter along thefirst and second paths, respectively, to combine the rays into an outputray pair; a compensator disposed between the beam splitter and the endmirror for equalizing the optical path lengths of the first and secondpaths; and a means for generating a difference between the first andsecond path lengths to create an interference pattern in the output raypair, the path length difference generating means disposed in the firstand second paths such that the first and second rays pass therethrough.29. An interferometer spectrometer as in claim 28, wherein translationaland rotational changes in relative position between the beam splitter,the compensator and the end mirror do not result in a lack ofparallelism between the rays forming the output ray pair.
 30. Aninterferometer spectrometer as in claim 29, wherein translationalchanges in relative position between the beam splitter, the compensatorand the end mirror do not result in a lateral separation of the raysforming the output ray pair.
 31. An interferometer spectrometer as inclaim 28, wherein translational changes in relative position between thebeam splitter, the compensator and the end mirror do not result in alateral separation of the rays forming the output ray pair.
 32. Aninterferometer spectrometer as in claim 28, further comprising a scannerplate of refractive material having parallel sides.
 33. Aninterferometer spectrometer as in claim 32, wherein the scanner plate isdisposed in both of the first and second paths.
 34. An interferometerspectrometer as in claim 33, wherein the first and second pathsintersect at an intersection point, and wherein the scanner plate isdisposed at the intersection point.
 35. An interferometer spectrometeras in claim 34, further comprising: an intersection mirror for causingthe first and second paths to intersect at the intersection point, theintersection mirror disposed between the beam splitter and the endmirror.
 36. An interferometer spectrometer as in claim 28, wherein thepath length difference generating means comprises rotation of the beamsplitter.
 37. An interferometer spectrometer as in claim 28, wherein thepath length difference generating means comprises rotation of thecompensator.
 38. An interferometer spectrometer as in claim 32, whereinthe path length difference generating means comprises rotation thescanner.
 39. An interferometer spectrometer as in claim 28, wherein theinput ray is provided by a light source, and wherein the light sourceproduces an elliptical angular subtense.
 40. An interferometerspectrometer as in claim 39, wherein the light source includes acollimator lens having an array of transmitting fibers disposed adjacentan array of receiving fibers.
 41. An interferometer spectrometer as inclaim 39, wherein the light source includes a plurality of collimatorlenses each having an array of transmitting fibers disposed adjacent anarray of receiving fibers.
 42. An interferometer spectrometer as inclaim 41, wherein the lenses are arranged in parallel.
 43. Aninterferometer spectrometer as in claim 39, wherein a single channel isutilized.
 44. An interferometer spectrometer as in claim 39, whereinmultiple channels are utilized.
 45. An interferometer spectrometer,comprising: a beam splitter comprising a block of refractive materialhaving parallel first and second surfaces, the second surface having apartially reflective material thereon for splitting an input ray into afirst ray and a second ray having a first path and a second path,respectively, the first surface having a reflective material thereon forreflecting the first ray such that the first and second rays exit thebeam splitter in parallel; an end mirror terminating the first path andthe second path to define a first path length and a second path length,respectively, wherein the end mirror reflects the first and second raysback to the beam splitter along the first and second paths,respectively, to combine the rays into an output ray pair; a compensatorcomprising a plate of refractive material having parallel sides, thecompensator disposed between the beam splitter and the end mirror forequalizing the optical path lengths of the first and second paths; and ameans for generating a difference between the first and second pathlengths to create an interference pattern in the output ray pair,wherein the path length difference generating means is common to boththe first and second paths.
 46. An interferometer spectrometer as inclaim 45, wherein translational and rotational changes in relativeposition between the beam splitter, the compensator and the end mirrordo not result in a lack of parallelism between the rays forming theoutput ray pair.
 47. An interferometer spectrometer as in claim 46,wherein translational changes in relative position between the beamsplitter, the compensator and the end mirror do not result in a lateralseparation of the rays forming the output ray pair.
 48. Aninterferometer spectrometer as in claim 45, wherein translationalchanges in relative position between the beam splitter, the compensatorand the end mirror do not result in a lateral separation of the raysforming the output ray pair.
 49. An interferometer spectrometer as inclaim 45, further comprising a scanner plate of refractive materialhaving parallel sides.
 50. An interferometer spectrometer as in claim49, wherein the first and second paths intersect at an intersectionpoint, and wherein the scanner plate is disposed at the intersectionpoint.
 51. An interferometer spectrometer as in claim 50, furthercomprising: an intersection mirror for causing the first and secondpaths to intersect at the intersection point, the intersection mirrordisposed between the beam splitter and the end mirror.
 52. Aninterferometer spectrometer as in claim 45, wherein the path lengthdifference generating means comprises rotation of the beam splitter. 53.An interferometer spectrometer as in claim 45, wherein the path lengthdifference generating means comprises rotation of the compensator. 54.An interferometer spectrometer as in claim 49, wherein the path lengthdifference generating means comprises rotation the scanner plate.
 55. Aninterferometer spectrometer, comprising: a beam splitter comprising aplate of refractive material having parallel leading and trailingsurfaces, the trailing surface having a partially reflective materialthereon for splitting an input ray into a first ray and a second rayhaving a first path and a second path, respectively, the leading surfacehaving a reflective material thereon for reflecting the first ray suchthat the first and second rays exit the beam splitter in parallel; anend mirror terminating the first path and the second path to define afirst path length and a second path length, respectively, wherein theend mirror reflects the first and second rays back to the beam splitteralong the first and second paths, respectively, to combine the rays intoan output ray pair; a compensator plate of refractive material havingparallel sides for equalizing the amount of refractive material in thefirst and second paths, the compensator plate disposed between the beamsplitter and the end mirror; an intersection mirror for causing thefirst and second paths to intersect at an intersection point, theintersection mirror disposed between the compensator and the end mirror;and a scanner plate of refractive material having parallel sides forgenerating a difference between the first and second path lengths tocreate an interference pattern in the output ray pair, the scanner platedisposed at the intersection point.
 56. An interferometer spectrometeras in claim 55, wherein translational and rotational changes in relativeposition between the beam splitter, the compensator and the end mirrordo not result in a lack of parallelism between the rays forming theoutput ray pair.
 57. An interferometer spectrometer as in claim 56,wherein translational changes in relative position between the beamsplitter, the compensator and the end mirror do not result in a lateralseparation of the rays forming the output ray pair.
 58. Aninterferometer spectrometer as in claim 55, wherein translationalchanges in relative position between the beam splitter, the compensatorand the end mirror do not result in a lateral separation of the raysforming the output ray pair.
 59. An interferometer spectrometer,comprising: a light source including an array of collimator lensesdefining an elliptical angular subtense; a beam splitter for splittingthe input ray into a first ray and a second ray having a first path anda second path, respectively; an end mirror terminating the first andsecond paths to define a first path length and a second path length,respectively; a compensator disposed between the beam splitter and theend mirror for equalizing the optical path lengths of the first andsecond paths; and a means for generating a difference between the firstand second path lengths.
 60. An interferometer spectrometer as in claim59, wherein each collimator lens has an array of transmitting fibersdisposed adjacent an array of receiving fibers.
 61. An interferometerspectrometer as in claim 59, wherein the lenses are arranged inparallel.
 62. An interferometer spectrometer as in claim 61, wherein asingle channel is utilized.
 63. An interferometer spectrometer as inclaim 61, wherein multiple channels are utilized.